1. Field of the Invention
The present invention relates generally to clock synchronization, and more particularly, to a clock synchronization protocol for performing stringent clock synchronization across multiple nodes.
2. Description of the Related Art
High precision clock synchronization is one of the most basic requirements in distributed real-time systems. Due to the unavoidable drift of local clocks, a global time base can only be achieved by means of a clock synchronization protocol.
The present invention, in its broadest application, is directed to a clock synchronization protocol for wirelessly synchronizing clock registers of nodes via a wireless local area network (e.g., 802.11 network). The present invention requires that systems employing the method of the invention operate in accordance with broadcast medium principles. That is, the present invention is intended for use in those systems in which a central or master node broadcasts messages (data) to a plurality of slave nodes in the network. Application of the method has potential widespread use whereby the nodes may be associated with any wired and/or wireless communication systems. For example, the nodes may be associated with well-known wired communication systems, such as Ethernet or 802.3. Alternatively, the nodes may be associated with wireless communication systems. For example, the principles of the present invention will be described herein in the context of a wireless embodiment including 1394 nodes wirelessly coupled via an 802.11 wireless local area network (WLAN). It is to be appreciated, however, that the wireless embodiment is a non-limiting exemplary embodiment.
Although a detailed description of the 1394 standard will not be provided herein, an overview of certain aspects of bus and node architecture will be presented to aid in understanding the present invention in a wireless context.
1394 Bus Overview
The IEEE 1394-1995 standard (also popularly known as “Firewire”™) describes a high speed, low cost serial bus to which compatible devices can be connected (even hot plugged) and automatically recognized. The IEEE 1394 standard defines a digital interface thereby eliminating the need for an application to convert digital data to analog data before it is transmitted across the bus.
IEEE 1394 currently defines a maximum wired distance or length of 4.5 meters. Thus, intrinsically, an IEEE 1394 serial bus can only be used to interconnect components which are relatively close together. Such a system of interconnected components which are close together is commonly referred to as a “cluster”. An exemplary cluster in a residential environment is a “multimedia island” such as a home entertainment cluster, a home computing cluster, a bedroom cluster, etc. The IEEE 1394 standard defines a simple mechanism to achieve clock synchronization between nodes in a single cluster.
It is possible to interconnect clusters, to extend their range via a wired or wireless connection. Currently, the P1394.1 working group makes the protocol standardization activities of a bridge for connecting two adjacent 1394 buses used in an IEEE-1394 high-speed serial bus communication environment, referred to as the 1394.1 bridge specification. As described above, high precision clock synchronization is required between the 1394 nodes in a single cluster, however, clock synchronization is also required between nodes across different clusters. Therefore, the specification defines a method of synchronizing two adjacent 1394 buses connected with a 1394.1 wired bus bridge.
Another approach to extending the IEEE 1394 serial bus is to create a wireless “bridge” between clusters. That may promise ease-of-use and the flexibility associated with an IEEE 1394 network. In general, it can be expected that wired IEEE 1394.1 bridges will be the pre-dominant approach within new residential buildings since IEEE 1394 “outlets” may be installed within every room. However, retrofitting wired IEEE 1394.1 bridges in existing residences may prove cost-prohibitive for many residences due to the extensive rewiring which will be required. Thus, for such existing residences, wireless IEEE 1394 bridges, if sufficiently low-cost, may provide a suitable alternative.
In support of the afore-mentioned approach for extending the IEEE 1394 serial bus via a wireless bridge, a method for synchronizing two or more 1394 buses, via wireless 1394 bridges, is defined in the ETSI TS 101 493-3 Specification, “Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Packet Based Convergence Layer; Part 3: IEEE 1394 Service Specific Convergence Sublayer (SSCS). In particular, the ETSI TS 101 493-3 specification defines a capability for providing synchronization between multiple 1394 buses connected via an HL/2 wireless area network (WLAN).
Accordingly, the present invention proposes a new clock synchronizing mechanism that can be implemented in the 802.11 environment.